Elastic spring elements are known and are used in many areas of engineering, primarily for damping vibrations and forces. The spring element here has a defined stiffness which is predetermined by the type, size, shape and number of the elastomer layers present and which, where appropriate, can be designed to be variable within certain limits after installation of the spring element.
In the case of large wind turbines with outputs of more than two megawatts, strong forces quite often act, in particular, on the rotor blades, rotor shaft and drive-train bearing arrangement, but also on the nacelle itself, and these forces subject the two-, three- or four-point bearings usually used for the gearboxes and generators to great loads, meaning that corresponding, in particular also vertical, displacements, deformations, distortions and pitch movements of the turbine can result, possibly with damage to the material or individual components.
The drive train of wind turbines is generally fastened elastically to the machine carrier. This is done on account of structure-borne noise insulation and also to enable displacements of the system. The known bearings are normally equipped with passive elastic elements which can absorb at least some of such constraining forces and generated vibrations owing to their alignment and different preset stiffnesses.
In the event of very high, in particular suddenly and rapidly occurring loads (extreme load case) especially in the vertical direction, i.e. normally in the direction in which the elastomer spring elements with the usual arrangement have high spring stiffness, the elasticity or stiffness of these elements, which, with respect to their adjustable stiffness, are generally designed for average loads and normally applied forces, in particular on the drive-train bearing arrangement, is not sufficient, so that the aforementioned deformations, displacements and distortions which occur in the turbine lead to damage to the turbine, especially to the gearbox, as described in more detail below.
In the prior art, various systems are employed for supporting the drive train of wind turbines. One of the systems is the moment bearing arrangement with rigid rotor shaft (FIGS. 1 and 2). In this system, the rotor shaft is connected to the machine carrier in a cardanically rigid manner, i.e. two rolling bearings (02) (movable bearing and fixed bearing), which transmit all the yawing and pitching moments, are seated on the rotor shaft. The advantage of this system is that all the loads arising at the rotor (01) are transmitted directly into the tower on a short path. A further advantage is that, on dismounting the rotor, it is possible for the rotor shaft (03) and the gearbox (04) to remain in the turbine. At the same time, the gearbox can also be dismounted without removal of the rotor shaft. The support can be provided as described by two rolling bearings or by one large moment bearing which performs the same mechanical function. In accordance with the prior art, the torque is supported on both sides of the gearbox. The disadvantage of this system is the indeterminate support resulting from four bearing points. This indeterminate support gives rise to constraining forces which may have the following causes: manufacturing and assembly tolerances, misalignment, inclination at the rotor shaft and at the gearbox flange and not least bending of all load-bearing elements relative to one another in the event of unsymmetrically acting force transmission by the rotor. Besides the drive torque, other moments additionally occur at the rotor. These are caused, for example, by uneven wind interference suppression or by the rotor blade moving past the tower.
In accordance with the prior art, elastomer components that are as soft as possible are used for the gearbox torque support, so that the displacement forces are kept as small as possible. However, this also causes a large rotary movement of the gearbox on load transmission, which in turn causes a displacement of the gearbox output shaft relative to the generator shaft, which is disadvantageous, and consequently this softness of the elastomer components can only be used to a limited extent, or accordingly complex couplings are required between gearbox and generator.
The system described in EP 1 566 543 A1 enables the constraining forces explained above to be removed from the four-point bearing arrangement of the turbine. This application describes an elastomer bearing arrangement for wind turbines with adjustable stiffness, in which the stiffness of the elastomer elements in the vertical direction can be varied by a hydraulic or mechanical device. These spring elements essentially consist of a connecting plate and an end plate between which is located at least one elastomer layer, the connecting plate having an opening with a connecting part, through which opening pressure can be exerted on the elastomer layer by a displacement element in the form of a hydraulic fluid or a movable piston element, with the result that an increase in prestress of the spring element and thus a stiffening in the vertical direction is achieved.
In practice, these spring elements, although achieving the desired technical effects in respect of the damping, have proved to be problematical since, under the requisite high pressures that have to be generated in order to increase the stiffness in the vertical direction sufficiently, leak-tightness problems repeatedly occur, with loss of hydraulic fluid, even though the elastomer layers compressed by hydraulic fluid are firmly connected to the surrounding parts by vulcanization and/or adhesive bonding.
After further investigations and tests with the spring element described in EP 1 566 543, but also generally in other elastomer bearing arrangements of the prior art, it has now been found, surprisingly, that large-volume elastomer components in particular cannot be readily sealed when they are in contact with hydraulic fluids of whatever type (e.g. water, oil, alcohols, mixtures of the same) under pressure. Small droplets of the hydraulic fluid are apparently absorbed, on continuous loading and under higher pressures, by the porous structure present, particularly in the case of large elastomer volumes, and are continuously transported further by the flow structures which are difficult to avoid in the elastomer material, until they escape at various, often unexpected, points of the component. Without wishing to be tied to a theory, these results may be interpreted in such a way that a droplet of the hydraulic fluid in the corresponding hydraulic space is forced into a small notch or pore in the surface of the adjacent elastomer which opens and closes during the dynamic loading, so that the droplet is locked in and continuously conveyed further until it reaches the end of the elastomer or component and thus results in a leak. The escape of hydraulic fluid from the spring elements or bearings of EP 1 566 543 A1 thus represents a serious problem which has to be solved.
Moreover, it has been found that direct contact of the hydraulic fluid with the elastomer material of the spring elements may lead to reduced durability or elasticity of the elastomer under the influence of high pressures, meaning that the corresponding spring elements may have to be replaced earlier.